In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator (SLM) and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator and detected with an array detector.
As described above, in page-oriented holographic data storage a pixelated spatial light modulator is used for modulating the object beam intensity with information. This intensity distribution is usually Fourier transformed by an objective lens. The Fourier transform, i.e. the spectrum of a pixelated data pattern has a high central intensity peak, hereafter referred to as DC-peak. The actual information is distributed around this peak on a much lower level, typically −60 dB. The DC-peak of the object beam can cause an undesired saturation of the photosensitive medium. The envelope of the surrounding intensity distribution can be described by a 2-dimensional sinc-function (sin (x)/x), which results from the usual square-like shape of the pixels. The full information about the SLM pixel pattern is located below the so-called Nyquist limit which lies at half the distance to the first zero of the sinc-function.
In order to suppress the DC-peak it has been proposed to apply a phase modulation in addition to the intensity modulation. For example, in M. J. O'Callaghan: “Sorting through the lore of phase mask options—performance measures and practical commercial designs”, Proc. SPIE Vol. 5362 (2004), pp. 150-159, different types of pixelated and non-pixelated phase masks are discussed. Typically, however, a binary phase mask is used for this purpose, which introduces a phase shift of 0 or π with respect to the laser wavelength. The phase cells, i.e. the areas with constant phase, have a size of one or more pixels of the SLM. The spatial distribution of 0 and π cells is random or pseudo random, the total number of 0 and π cells is essentially the same.
A phase mask with a cell size of one SLM pixel suppresses the DC-peak quite well. However, a large fraction of intensity is still located above the Nyquist limit. This fraction is redundant and does not contain necessary data information. Furthermore, because of the sinc-like envelope, the intensity distribution is not flat within the central region of the Fourier plane or in the holographic medium, respectively.
This drawback is overcome by a phase mask having a phase modulation with a lower spatial resolution than the SLM. Such a phase mask lead to a narrower intensity distribution in the Fourier plane. Therefore, it reduces the unnecessary intensity above the Nyquist limit, whereas the important intensity below the Nyquist limit is increased.